Long Option Parsing in C++

LongOptionsParse.cpp

#include <getopt.h>
#include <iostream>

using namespace std;

void usage(const char* appName, int exitCode, string errMsg="")
{
   if (!errMsg.empty())
      cout<<"\n"<<errMsg<<endl;
   cout<<"\nProgram description goes here..."
         "\n"
         "\nUsage:  "<<appName<<" [options] <input-file>"
         "\n"
         "\nOptions:"
         "\n  -h                            Print this usage and exit."
         "\n  -o, --output-file <filename>  Specify name of output file."
         "\n  --rational-value <float>      Provide an optional double value."
         "\n  --version                     Output version information and exit."
         "\n"
         "\nExample:  "<<appName<<" -o my-output.dat my-input.dat"
         "\n"<<endl;
   exit(exitCode);
}

int main (int argc, char **argv)
{
   int c, versionFlag=0, optIndex=0;
   double rationalValue=0.0;
   string optName;
   string inputFile, outputFile;

   // Long options specified here as { <name>, <has-arg>, &<my-var>, <value> }, where:
   // <name>     long option name, e.g. "version"
   // <has-arg>  can be: no_argument | required_argument | optional_argument
   // <my-var>   int variable to be assigned to <value> if option present. If NULL, then nothing is
   //            assigned and it is up to the switch statement to read possible argument in optarg
   //            global variable. Alternatively, if NULL, and <value> set to the option's single
   //            character equivalent, getopt_long will behave as if the single character was
   //            specified. If <my-var> is used, variable must be pre-assigned to its default value.
   // <value>    If option present, value assigned to <my-var> if my-var is not null. Usually just
   //            a boolean 0 | 1. if <my-var> *is* null, then <value> can contain the single-char
   //            option and it will be returned as such by getopt_long.
   static struct option long_options[] = {
      { "output-file", required_argument, 0, 'o' },  // filename found in optarg, parsed as 'o' option
      { "rational-value", required_argument, 0, 0 }, // value found in optarg
      { "version", no_argument, &versionFlag, 1 },   // versionFlag is set to 1
      { 0, 0, 0, 0}                                  // always last
   };

   while ((c = getopt_long(argc, argv, "ho:", long_options, &optIndex)) != -1)
   {
      switch (c)
      {
      case 0:
         // Indicates long option detected, use option index to access:
         if (long_options[optIndex].flag != 0)
            break; // value set in flag variable (e.g., versionFlag)

         // Process long option that doesn't have a short option equivalent:
         optName = long_options[optIndex].name;
         if (optName == "rational-value")
            rationalValue = atof(optarg);
         break;
      case 'h':
         usage(argv[0], 0);
         break;
      case 'o':
         outputFile = optarg;
         break;
      case '?':
         // getopt_long already printed an error message.
         usage(argv[0], 1);
         break;
      default:
         usage(argv[0], 1, "Fatal parsing error occurred");
      }
   }
   if (versionFlag)
   {
      cout<<"\nSoftware version 0.0.1\n"<<endl;
      exit (0);
   }

   // Handle any remaining command line arguments (not options):
   if (optind < argc)
      inputFile = argv[optind++];
   else
      usage(argv[0], 1, "Input filename required.");

   // Any additional items are superfluous:
   if (optind < argc)
      usage(argv[0], 1, "Extra command line arguments detected!");

   // Proceed with application:
   cout<<"inputFile: "<<inputFile<<endl;
   cout<<"outputFile: "<<outputFile<<endl;
   cout<<"rationalValue: "<<rationalValue<<endl;

   exit (0);
}

This gist is intended to be inserted into an application's main.cpp